More than a thousand people living on the Big Island of Hawaii have been evacuated after the region's Kilauea volcano began erupting on Thursday. By early Friday, the eruptions from the volcano's east rift zone were shooting lava more than a hundred feet into the air, and rivers of lava were creeping into a nearby subdivision, Hawaii News Now reports.

The activity is alarming but not surprising. In the days leading up to the eruption, scientists from the U.S. Geological Survey registered hundreds of small quakes in their monitoring systems—signs that an eruption was likely imminent. What's more, this volcano has been active since 1983, making it one of the longest erupting volcanoes in the world.

In general, eruptions at Kilauea are famous for slow-moving, river-like lava flows called effusive eruptions, and the national park in which the volcano sits is a popular tourist attraction. So what are the biggest threats a Kilauea eruption presents today, and what kind of activity can residents expect from the volcano in the future?

A River of Lava

“The major threat is lava inundation,” says volcanologist Janine Krippner from Concord University. Because this eruption sprang up in an inhabited region, homes have become sitting ducks.

Despite being a well studied and monitored peak, predicting where and when a Kilauea eruption will occur is difficult. Scientists can see the signs of an eruption in the days preceding it—cracks begin to form and seismic activity ramps up. But “it's hard to say how long it will last, because the magma is coming up from so deep,” adds Krippner.

Sulfur dioxide released in the volcanic gas accompanying an eruption can also present a danger. The lava flows are runny, meaning gasses can easily escape, Krippner notes. Inhaling large amounts of the gas can irritate a person's eyes, nose, and throat, and for those with respiratory conditions, the gas can make breathing even more difficult.

When rain is in the forecast, volcanic gas can lead to acid rain, but the sulfuric acid isn't actually concentrated enough to physically damage a human. Breathing in the fumes presents a more serious health threat. Acid rain can, however, seep into the soil, making it more difficult for plants to get the nutrients they need.

A (Possibly) Violent Future

A bigger threat, though, may loom over Kilauea. A study published in 2009 revealed that the volcano once had a violent past. By looking at volcanic rock and ash, or tephra, lying miles away from the volcano, scientists surmised that an explosive eruption sent material flying through the air roughly 1,000 to 1,600 years ago. Another large but less intense eruption seems to have rocked the island around 500 to 200 years ago.

“It does happen, and it could happen, but it's quite rare,” Krippner says of these explosive types of eruptions.

Smaller explosive eruptions can occur when rock falls into the crater. And hot magma coming into contact with cool water caused a deadly explosion in 1924, when the resulting steam built up so much pressure that the volcano burst. Krippner thinks these kinds of random, freak explosions are the most likely violent outbursts we can expect from Kilauea for the foreseable future.